Strolling down the wide, grass-lined street, after passing the Burger King on the left and the convenience store on the right, you would be forgiven for thinking you are in Iowa, but the Yongsan Garrison is in the geographic heart of one of the most densely populated cities in the world: Seoul. Picture putting walls up around Central Park and handing it over to the German army.

Yongsan (Dragon Mountain), and the more than 80 other American bases scattered throughout South Korea, was built in the wake of a war that ended 55 years go. Given the Korean high-rise apartments and office buildings that now surround the compound (rendering the base indefensible if war were to actually break out) and the deepening economic cooperation with the North, the dilapidated facilities are an anachronism. Throw in occasional tragic accidents and crimes committed by US soldiers, and you seem to have a recipe for "Yankee go home!"

Yet public opinion surveys consistently show that South Koreans, by a solid 2-1 margin, want Uncle Sam to stay. Despite a decade of cooperation, the average South Korean not only harbors a healthy mistrust of the North, but is equally wary of China and Russia, and its former colonial oppressor, Japan. Given these neighbors, even the Bush years seemed tolerable.

Beyond the security assurance itself, it is the military resources that America's 26,000 troops bring to the Korean table that makes the Untied States so indispensable. It would take at least a decade—and billions of dollars—to replace the air and intelligence-gathering capabilities alone that South Korea would lose should America pull out. For the US, beyond the moral obligation we have to Koreans after dividing their country back in 1945, Russia and China's close proximity provide ample strategic reasons to maintain forward deployed troops, even in a unified Korea.

Not that everything is peaches and cream. In fact, there are a range of contentious issues that must be resolved if the American military presence in Korea is to be sustained, starting with the Yongsan Garrison. Since at least the 1980s, there has been a general recognition of the need to relocate this US military headquarters, but Seoul and Washington have been dragging their feet on determining how to divvy the $10 billion tab. The relocation of Yongsan 50 miles south of Seoul is part of a broader, ongoing consolidation that will reduce the number of bases from 87 to 10.

The two governments are also haggling over burden-sharing more broadly. Washington wants Seoul to raise its financial contribution to the stationing of US troops from 41 percent to half; Seoul insists that by its calculations, Korea has already reached that level. Expect a compromise to be hammered out later this year.

In many ways, President Bush and South Korea's former president, Roh Moo-hyun, were the ultimate test for the military alliance. Though the same age and equally careless with words, they came from opposite ends of the political spectrum and never managed to do any more than tolerate one another. Their subordinates often made things worse.

It is normal for allies of powerful countries to sometimes feel abandoned or entrapped by their patron, but in 2004 Secretary of Defense Donald Rumsfeld pulled off that rare feat of making Seoul feel both of these sentiments when he decided—unilaterally—to reduce US troop presence and pull back the remaining troops from the demilitarized zone. Seoul interpreted this as a weakening of the US commitment and a move that could facilitate a US strike against North Korea by removing American forces from the North's artillery range.

Seoul was equally uncomfortable with the Bush administration's insistence on strategic flexibility, which makes it easier for Korea-based American soldiers to participate in other potential conflicts. Second only to Seoul's fear of a preemptive strike on the North is the prospect of being pulled into a conflict with China.

President Roh made matters worse by demanding the return of operational control of Korean forces during wartime. (Seoul regained peacetime control from the US in 1994.) Rummy was more than happy to oblige. After much wrangling, the two sides agreed on 2012 for the reversion, but as one Korean general told me, "There is no way we can be ready by then." While the Pentagon has asserted that the deadline cannot be renegotiated, expect an extension as it approaches.

South Korea will remain a pillar of the US military presence in Asia, but Bush and Roh's successors have some repair work to do.